Only the applicant can petition to reinstate and they need to establish that the delay in paying was unintentional and account for the whole time period. This includes the period from when the applicant realizes the patent expired and the petition was filed.
The USPTO might not seriously challenge the assertions made in petition but, if it is reinstated and the patent owner attempts to enforce it they will find the patent weakened if much time has occurred after the grace period. Disproving the unintentional delay might be a target of a defense in court.
2590 Acceptance of Delayed Payment of Maintenance Fee in Expired Patent to Reinstate Patent
In the section of the MPEP on petitioning for reinstatement it says, if granted by the Director, it is as if it never lapsed. But in important respects that is not operational.
2591 Intervening Rights in Reinstated Patents says that anything that would be infringing done after the 6 month grace period and before it was reinstated has certain rights and that a court may let activities started then to continue after reinstatement.